AMERICIUM 103 4. CHEMICAL, PHYSICAL, and RADIOLOGICAL INFORMATION 4.1 CHEMICAL IDENTITY Americium is a human-made actinide element (atomic number 95) and has no stable isotopes. It was discovered by Glen Seaborg, Leon Morgan, Ralph James, and Albert Ghiorso in 1944 and isolated by 241 B.B. Cunningham as the isotope Am in Am(OH)3 in the fall of 1945. It was named after the Americas (Seaborg 1991; Seaborg and Loveland 1990). Actinides are the 15 elements starting with actinium, atomic number 89, and extending to lawrencium, atomic number 103. All of the isotopes of these elements are radioactive. Of the 15 americium isotopes and isomers currently identified, the longest-lived is 243Am. 4.2 PHYSICAL, CHEMICAL, AND RADIOLOGICAL PROPERTIES Americium is a silvery, ductile, very maleable, non-magnetic metal. Americium melts at 1,176 ºC, boils at 2,011 ºC, and has an electron configuration of 5f77s2. The Chemical Abstract Service (CAS) registry numbers, decay modes, half-lives, and specific activities of the four principal americium isotopes and isomers, 241Am,242mAm, 242Am, and 243Am, are presented in Table 4-1. 241Am and 243Am decay by alpha 237 6 239 emission forming neptunium (atomic number 93), Np (t½=2.14x10 years) and Np (t½=2.355 days) as 239 239 4 235 the respective products. Np subsequently decays to Pu (t½=2.411x10 years), and then to U, which is also a naturally-occurring isotope of uranium. The decay of 241Am to 237Np is accompanied by a predominant gamma ray photon of 59.54 keV. 242mAm is an isomer (long-lived excited state of the nucleus) of 242Am and has a half-life of 141 years; 95.5% of 242mAm undergoes an isomeric transition to 242 Am (t½=16.02 hours) with the emission of a 48.6 keV gamma ray, and 0.5% decays by alpha emission to 238Np. 242Am primarily (87%) undergoes beta decay to curium 242 (atomic number 96) 238 234 5 (t½=162.8 days), which decays to Pu (t½=87.74 years) and then to U (t½=2.45x10 years) (another 242 5 naturally-occurring uranium isotope); 17% undergoes electron capture to Pu (t½=3.76x10 years). 241Am has a high specific activity of 3.428 Ci/g (0.1268 TBq/g), emitting ~7x109 alpha particles/mg/ minute. 243Am has a specific alpha activity about 17 times lower than 241Am and is therefore more attractive for chemical investigations of the element (ICRP 1983; Lide 1998; O’Neil 2001; Seaborg 1991). AMERICIUM 104 4. CHEMICAL, PHYSICAL, AND RADIOLOGICAL INFORMATION Table 4-1. Principal Americium Isotopes Particle energy CAS Decay mode/ Specific Gamma Registry percent Decay mode Energy Intensity Half-life activityb energy Isotopea No. (product) energy(MeV) (MeV) (percent) (years) (Ci/g) (keV) 241 23 7 Am 14596-10-2 " ( 93 Np) 5.637 5.4431 12.8 432.2 3.43 26.34 86954-36-1c 5.4857 85.2 33.192 5.3884 1.4 59.536 Others <1 242mAm 13981-54-9 I.T./95.5 0.048 141 10.5 48.63 (242Am) 23 8 " /0.5( 9 3Np) 5.62 5.141 0.026 86.48 5.2070 0.4 109.44 163.04 242 - 24 2 Am 13981-54-9 $ /83 ( 9 6Cm) 0.665 0.63 46 16.02 808,000 42.2 hours 24 2 EC/17 ( 9 4Pu) 0.750 0.67 37 44.53 243 23 9 Am 14993-75-0 " ( 93 Np) 5.438 5.1798 1.1 7,370 0.199 43.54 5.2343 11 74.67 5.2766 88 86.57 Others <1 117.70 141.97 aAn m after the atomic mass indicates one of multiple isomers of a given atomic mass. bLANL 1999 (1 Ci=0.037 TBq) cCAS Registry Number for 241Am3+ ion Source: Baum 2002; DOE 1997b; ICRP 1983; LBL 2000; Lide 1998 AMERICIUM 105 4. CHEMICAL, PHYSICAL, AND RADIOLOGICAL INFORMATION The known oxidation states of americium are +2, +3, +4, +5, and +6. However, the stable oxidation states are +3 and +4; the common oxidation state is +3, in which state, the behavior of americium and other actinides is similar to the lanthanides. The trivalent state is the only state of importance in biological systems. The +2 oxidation state is very unstable and has only been produced in solid compounds. The stability of the americium oxidation states higher than +3 is less than that of uranium, neptunium, and plutonium (Cotton and Wilkinson 1980; Nenot and Stather 1979; Seaborg 1991). Am+3 hydrolyzes and forms weak complexes with serum proteins and other ligands. The physical and chemical properties of americium and selected americium compounds are shown in Table 4-2. Properties of some americium ions are shown in Table 4-3. The decay schemes for 241Am and 243Am are shown in Tables 4-4 and 4-5. AMERICIUM 106 4. CHEMICAL, PHYSICAL, AND RADIOLOGICAL INFORMATION Table 4-2. Physical and Chemical Properties of Americium and Selected Americium Compounds Value Americium(III) Americium(III) Property Americium Americium(II) oxide oxide chloride Atomic/molecular weighta 243 275 534 349 Chemical formula Am AmO2 Am2O3 AmCl3 Synonyms No data No data No data No data Chemical Abstracts 7440-35-9b 12005-67-3 12254-64-7 13464-46-5 Service Registry No. Color Silvery Black Tan Pink Physical form Solid metal Cubic crystals Hexagonal Hexagonal crystals crystals Odor No data No data No data No data Melting point, EC 1,176 >1,000 (decomposes) No data 500 Boiling point, EC 2,011 Not relevant No data No data Autoignition temperature No data Not relevant Not relevant Not relevant Solubility: Water Insoluble No data No data Soluble Other solvents Soluble in acid Soluble in acid Soluble in acid No data Density, g/cm3 12 11.68 11.77 5.87 Partition coefficients No data No data No data No data Vapor pressure No data No data No data No data Refractive index No data No data No data No data AMERICIUM 107 4. CHEMICAL, PHYSICAL, AND RADIOLOGICAL INFORMATION Table 4-2. Physical and Chemical Properties of Americium and Selected Americium Compounds Value Property Americium(III) fluoride Americium nitrate Americium citrate Atomic/molecular weighta 300 429 432 Chemical formula AmF3 Am(NO3)3 AmC6H5O7 Synonyms No data Americium trinitrate No data Chemical Abstracts Service 13708-80-0 25933-53-3 11078-88-9 Registry No. Color Pink No data No data Physical form Hexagonal crystals No data No data Odor No data No data No data Melting point, EC 1,393 No data No data Boiling point, EC No data No data No data Autoignition temperature Not relevant Not relevant Not relevant Solubility: Water No data No data No data Other solvents No data No data No data Density, g/cm3 9.53 No data No data Partition coefficients No data No data No data Vapor pressure No data No data No data Refractive index No data No data No data aCalculated for 243Am bThis is also a generic CAS Registry Number for americium (unspecified form). Source: Chemical Abstract Service 2000; Lide 1994, 1998, 2000 AMERICIUM 108 4. CHEMICAL, PHYSICAL, AND RADIOLOGICAL INFORMATION Table 4-3. Properties of Some Americium Ions Ion Color Stability Am3+ Pink or yellow Stable; difficult to oxidize Am4+ Color unknown Occurs in solution only as complex fluoride and carbonate ions + AmO2 Yellow Disproportionates in strong acid; reduces fairly rapidly under action of own alpha emissions (241Am) at low acidification 2+ AmO2 Rum colored Easily reduced; reduces fairly rapidly under the action of its own alpha radiation (241Am) Source: Seaborg 1991 AMERICIUM 109 4. CHEMICAL, PHYSICAL, AND RADIOLOGICAL INFORMATION Table 4-4. 241Am Decay Schemea Energies and intensities of emitted radiation Alpha (α) Beta (β) max Gamma (γ) Nuclide Half-life keV % MeV % keV % 241Am 432.2 years 5,485 84.5 13.9 42 ↓ 5,443 13.0 59.5 35.9 26.3 2.4 237Np 2.144x106 4,788 47 13.3 58 ↓ years 4,771 25 29.4 15 4,766 8 86.5 12.4 4,639 6.2 8.2 9 4,664 3.3 95.9 2.7 233Pa 26.97 days 232 40 13.6 56 ↓ 156 27.7 312.2 38.6 260 17 98.4 17.7 174 16.4 94.7 10.9 572 4 111.0 8.2 233U 1.592x105 4,824 84.4 13.0 6.0 ↓ years 4,784 13.2 229Th 7,340 years 4,845 56.2 12.3 79 ↓ 4,901 10.2 88.5 24.7 4,815 9.3 85.4 15.0 5,053 6.6 100.0 11.3 4,968 6.0 11.1 8 225Ra 14.9 days 331 69.5 40.0 30.0 ↓ 371 30.5 12.7 15.2 225Ac 10.0 days 5,830 50.7 12.0 20.9 ↓ 5,792 18.1 10.6 9.3 5,791 8.6 5,732 8.0 5,637 4.4 221Fr 4.9 minutes 6,341 83.4 218.2 11.6 ↓ 6,126 15.1 11.4 2.2 217At ↓ 0.323 seconds 7,066 99.9 258.5 0.056 213Bi 45.59 minutes 5,869 1.94 1422 65.9 440.5 26.1 97.84% ↓ 2.16% 982 31.0 79.3 2.0 213Po 3.65 micro- 8,376 100.0 778.8 0.005 ↓ seconds 209Tl 2.2 minutes 660 98.8 1567.1 99.8 ↓ 465.1 96.9 117.2 84.3 75.0 10.7 10.6 9.4 209Pb ↓ 3.253 hours 0.644 100 209Bi stable aMinimum intensity 2%, up to five energies, with at least one entry per radiation type (DOE 2003) AMERICIUM 110 4.